In general, in-line optical isolators are used in optical communication systems and optical sensor systems to prevent the reflected optical signal of transmitted optical signals from being input into the signal processing parts of optical transmission equipment. Because the state of polarization of transmitted optical signals normally does not stay constant, a polarization-independent optical isolator is required.
Conventionally, optical isolators of this type include, for example, a polarization-independent optical isolator disclosed in Patent Literature 1 below. This polarization-independent optical isolator uses a birefringent plate to isolate transmission in the reverse direction, and is comprised of, arranged in the stated order, a birefringent plate, a first and a second reversible rotating means, an irreversible rotating means, a lens and a concave mirror.
In this configuration, an input optical signal, which enters from the input optical fiber and propagates in the forward direction, enters the birefringent plate and is separated into an ordinary ray (O ray) and an extraordinary ray (E ray) in a plane including the optical axes of the input optical fiber and the output optical fiber. The separated rays then pass through the first reversible rotating means, but it does not affect the rotating angle of the polarizing directions of the rays. Next, the rays enter the Faraday rotator, which is an irreversible rotating means, where the polarizing directions are rotated counterclockwise by 22.5 degrees. The rays then collide with a reflecting means consisting of a lens and a concave mirror, and reenter the Faraday rotator after the reflection. At this time, the polarizing directions of the rays are again rotated counterclockwise by 22.5 degrees, due to the irreversibility of the Faraday rotator. It should be noted that the spatial positions of the rays are interchanged by this reflection, but the state of polarization of each ray remains the same as before the reflection.
Next, the rays pass through the second reversible rotating means, and the polarizing directions of the rays are rotated counterclockwise by 45 degrees. As a result, the sum of rotating angles of the polarizing directions of the rays comes to 90 degrees, which means that the state of polarization and spatial positions of the rays have been interchanged. Then, the rays enter the birefringent plate, and the original ordinary ray propagating in the direction of the extraordinary ray recombines with the original extraordinary ray propagating in the direction of the ordinary ray due to the spatial shift effect, and the input optical signal is input into the output optical fiber. Also, in the reverse direction (the direction of isolation), the Faraday rotator and the pair of irreversible rotating means make the sum of rotating angles of the polarizing directions of the rays 0 degrees. Thus, the rays that are input from the output optical fiber are not recombined at the birefringent plate and therefore are not input into the input optical fiber.
Conventionally, there also exists a type of polarization-independent optical isolator, which is disclosed in Patent Literature 2 below. This optical isolator is comprised of, arranged in the stated order, an optical fiber array, a rutile crystal (a birefringent plate), a half-wave plate and a glass plate (a reversible rotating means), a rod lens, a garnet crystal (an irreversible rotating means) and a reflecting mirror. The half-wave plate and the glass plate are arranged so that each covers one half of the area of the rod lens.
In this configuration, an optical signal, which enters from the input optical fiber and propagates in the forward direction, first enters the rutile crystal and is separated into an ordinary ray and an extraordinary ray in a plane including the optical axes of the input optical fiber and the output optical fiber. The separated rays then pass through the glass plate and enter the rod lens, where they are converted to parallel rays. After passing through the rod lens, the separated rays enter the garnet crystal, which is a Faraday rotator, where their polarizing directions are rotated counterclockwise by 22.5 degrees. Next, the rays are reflected by the reflecting mirror, and the spatial positions of the rays are interchanged. The rays reflected by the reflecting mirror reenter the garnet crystal and their polarizing directions are again rotated counterclockwise by 22.5 degrees due to the irreversibility of the garnet crystal. Then, by passing through the half-wave plate after passing through the rod lens, the polarizing directions are further rotated counterclockwise by 45 degrees.
As a result, the sum of rotating angles of the polarizing directions of the rays comes to 90 degrees, which means that the state of polarization and the spatial positions of the rays have been interchanged. Then, the rays enter the rutile crystal, in which the original ordinary ray propagating in the direction of the extraordinary ray recombines with the original extraordinary ray propagating in the direction of the ordinary ray due to the spatial shift effect, and is input into the output optical fiber. Also, in the reverse direction (direction of isolation), the garnet crystal and the half-wave plate are used to make the sum of rotating angles of the polarizing directions of the rays 0 degrees. Thus, the ordinary ray and the extraordinary ray are not recombined in the rutile crystal and therefore are not input into the input optical fiber.
When the optical signal path transmits multiple optical signals, an arrayed optical isolator having multiple input/output ports is used.
Conventionally, this type of arrayed optical isolators include an arrayed optical isolator in which multiple polarization-independent optical isolators are integrated, and a polarization-independent optical isolator array which is disclosed in Patent Literature 3 below. In this polarization-independent optical isolator array, a multifiber optical fiber array (FA) equipped with a lens is provided at the input port and the output port, and two birefringent crystal plates (BP), two Faraday rotators (FR) and multiple polarizing plates (PR) are provided between them to isolate transmission in the reverse direction. In this configuration, an input optical signal, which is input from the input optical fiber and propagates in the forward direction, is output from the output optical fiber but is not input into the input optical fiber in the reverse direction (direction of isolation).
However, the polarization-independent optical isolator disclosed in Patent Literature 1 has four problems, which are described below.
Firstly, because the ordinary ray and the extraordinary ray that are separated by the birefringent plate and the ordinary ray and the extraordinary ray that are reflected by the reflecting means all exist in a plane that includes the optical axes of the input optical fiber and the output optical fiber, the optical path length where the ordinary ray propagates along the central optical axis of the lens is different from the optical path length where the extraordinary ray propagates along the central optical axis of the lens. Thus, the time it takes to propagate to the birefringent plate, where recombination occurs, differs between the ordinary ray and the extraordinary ray, and the dispersion phenomenon of optical signals due to various polarization (polarization mode dispersion) occurs. For example, when a 450 [μm]-thick rutile crystal is used as a birefringent plate, a difference in propagation time of about 0.5 [psec] is estimated between the ordinary ray and the extraordinary ray. This will result in constraints when applying this optical isolator to high-speed optical transmission equipment with transmission rates of 10 [Gb/s] or higher, such as the number of optical isolators that can be used.
Secondly, each optical axis must be accurately aligned at a predetermined angle between the birefringent plate and the reversible rotating means, in order to ensure low insertion loss and high isolation of the optical signal. However, when a pair of reversible rotating means with different directions of optical axis was used, the angles of optical axis had to be adjusted twice. In other words, the polarizing direction of the optical signal, which enters from the input optical fiber and is separated by the birefringent plate, and the direction of the crystal optical axis of the first reversible rotating means must be accurately aligned and, furthermore, the direction of the crystal optical axis of the second reversible rotating means must be accurately aligned at a predetermined angle so that the polarizing direction of the reflected optical signal which is input into the birefringent plate coincides with the composite direction of the birefringent plate.
Thirdly, a large interval of about 250 [μm] was required between the optical axes of the input and the output optical fibers in order to align the optical fibers.
Fourthly, in this configuration, the optical signal, after entering from the input optical fiber, passes through the birefringent plate, the reversible rotating means and the Faraday rotator, before passing through the lens. This causes the incident light from the input optical fiber to diffuse, and the effective diameter of the lens had to be increased. Therefore, it was difficult to reduce the size of the optical isolator.
Also, in the optical isolator disclosed in Patent Literature 2, the angle of optical axis of each optical device is adjusted in advance, and the optical devices are mutually adhered and fixed. Therefore, the second problem associated with the polarization-independent optical isolator disclosed in Patent Literature 1 does not occur. Also, in the optical isolator disclosed in Patent Literature 2, the optical fibers are arrayed and the incident light from the input optical fiber enters the Faraday rotator and the reflecting mirror after passing through the rod lens. Therefore, the third and fourth problems associated with the polarization-independent optical isolator disclosed in Patent Literature 1 do not occur.
However, even in the optical isolator described in Patent Literature 2, a dispersion phenomenon of optical signals caused by ordinary ray and extraordinary ray (polarization mode dispersion) occurs. For example, when a 1300 [μm]-thick rutile crystal is used, a difference of about 1.2 [psec] is estimated between the ordinary ray and the extraordinary ray in the time required to propagate to the rutile crystal where recombination occurs. Due to this difference in propagation time, there will be constraints when applying this optical isolator to high-speed optical transmission equipment with transmission rates of 10 [Gb/s] or higher, such as the number of optical isolators that can be used.
Also, when excited light that has a wavelength of 0.98 [μm] is input in the optical isolator described in Patent Literature 2, iron component, which is the material for the Faraday rotator, absorbs the 0.98 [μm] light and generates heat. As a result, at the fixed adhesion interface between the Faraday rotator and the adjacent rod lens or the reflecting mirror, adhesive degenerate due to rise in temperature and its characteristics change in some cases.
Also, the above conventional optical isolator arrays with integral configuration, comprised of multiple polarization-independent optical isolators, simply increases in size according to the number of integrated optical isolators. Also, in the above conventional optical isolator arrays disclosed in Patent Literature 3, multiple polarizing plates (PR) must be stacked vertically according to the number of optical isolator functions, which causes the functional components, (BP) and (FR), which integrate the functions common to multiple optical isolators, to increase in size. As a result, conventional optical isolator arrays also have tended to be large in size. Also, to increase the number of input/output ports, polarizing plate (PR) must be replaced to meet the increase in the number of ports, which requires time and effort.
Patent Literature 1: Patent No. 2710451 Gazette (Page 3, right column, line 14 to page 4, right column, line 15, FIG. 1)
Patent Literature 2: Japanese Publication of Unexamined Patent Application No. 5-313094 Gazette (paragraphs [0010]-[0014], FIG. 1 and FIG. 2)
Patent Literature 3: Japanese Publication of Unexamined Patent Application No. 5-188324 Gazette (paragraphs [0007]-[0010], FIG. 1 and FIG. 3)